Part of the problem of evil is the problem of why Satan is given so much freedom to harm the world, when God has the right and power to throw him in the pit. God will one day do away with Satan altogether (Revelation 20:3, 10). That will be no injustice to Satan. Nor would it be unjust for God to do it today. So why doesn't He, in view of how much misery Satan causes?
Satan roams like a devouring lion to destroy faith (1 Peter 5:8); he makes people sick and diseased (Acts 10:38); he tempts to sin (Luke 22:3-4); he blinds the minds of unbelievers (2 Corinthians 4:4); he takes people captive to do his will (2 Timothy 2:26); he kills (Revelation 2:10). One day God will stop him from doing this. Why doesn't He stop him now?
Could it be that there is a chance the devil and his angels will repent? Is God is giving them time? No. The Bible teaches they are irredeemable. Jesus said that "the eternal fire . . . has been prepared for the devil and his angels" (Matthew 25:41). Jude confirms this when he says that the fallen angels are being "kept in eternal bonds under darkness for the judgment of the great day" (Jude 1:6).
Why then does God tolerate Satan? The key is that God aims to defeat Satan in a way that glorifies not only his power, but also the superior beauty and worth and desirability of his Son over Satan. God could simply exert raw power and snuff Satan out. That would glorify God's power. But it would not display so clearly the superior worth of Jesus over Satan. That will be displayed as Christ defeats Satan by his death and then by winning superior allegiance from the saints over the lies of Satan.
Central to this plan is that God defeats Satan in stages through the work of Christ. Paul says that when we were forgiven all our trespasses by Christ's death on the cross, God thus "disarmed the rulers and authorities and put them to open shame, by triumphing over them in him" (Colossians 2:15).
This was the first stage of Satan's defeat. How was he defeated by the cross? The lethal weapon of soul-destroying sin and guilt is taken out of Satan's hand. He is disarmed of the single weapon that can condemn us - unforgiven sin. We see this in 1 Corinthians 15:55-57, "O death, where is thy victory? O death, where is thy sting? The sting of death is sin, and the power of sin is the law. But thanks be to God, who gives us the victory through our Lord Jesus Christ."
Without sin and law to condemn us, Satan is a defeated foe. He is disarmed. Christ has triumphed over him, not yet by casting him into hell and nullifying his influence on earth, but by letting him live and watch while millions of saints find forgiveness for their sins and turn their back on Satan because of the greater glory of Christ.
That is a second stage of defeat: the conversion of people by the power of the gospel of the cross. Jesus says to Paul that his mission to the Gentiles is "to open their eyes, so that they may turn . . . from the power of Satan to God" (Acts 26:18). This is what happens when God removes the blindness caused by the devil and gives us the light of the gospel of the glory of Christ (2 Corinthians 4:4-6). This enables people to see the ugliness of Satan and the beauty of Christ so that their choosing Christ glorifies not only God's power, but Christ's superior beauty and worth over Satan.
This way of defeating Satan is a costly triumph. Christ suffered and the world suffers. But God's values are not so easily reckoned. If Christ obliterated all demons now (which he could do), His sheer power would be seen as glorious, but His superior beauty and worth would not shine so brightly as when God's people renounce the promises of Satan, trust in Christ's blood and righteousness, and take pleasure in the greater glory of Jesus over Satan.
This means that our treasuring Christ above all the promises of sin and Satan is part of the triumph of that God designs for this age. Take up arms! Be glad in the Son of God!